THE HUMAN EYE AND THE COLOURFUL WORLD
SCATTERING OF LIGHT
The path of a beam of light passing through a true solution is not visible. But it is visible through a colloidal solution due to scattering of light.
Tyndall Effect
When a beam of light strikes
particles in the atmosphere, the path of the beam becomes visible. The light
reaches us, after being reflected diffusely by particles.
Tyndall effect
is the phenomenon of scattering of light by the colloidal particles or very fine
suspension. E.g.
o When
a beam of sunlight enters a smoke-filled room, the particles become visible due
to scattering of light.
o When
sunlight passes through a canopy of a dense forest, tiny water droplets in the
mist scatter light.
The colour of the scattered
light depends on the size of the scattering particles. Very fine particles
scatter mainly blue light while large sized particles scatter light of longer
wavelengths. If the size of
particles is larger, the scattered light appears white.
Why is the colour of the clear Sky Blue?
Size of the molecules and
particles in the atmosphere is generally smaller than the wavelength of visible
light. So, light of shorter wavelengths at the blue end is more scattered than light
of longer wavelengths at the red end.
The red light has a wavelength
about 1.8 times greater than blue light. Thus, when sunlight
passes through the atmosphere, the fine particles in air scatter the blue
colour more strongly than red. The scattered blue light enters the eyes.
If the earth had no atmosphere,
there would be no any scattering. Then, the sky would have looked dark. The sky appears dark to passengers flying at
very high altitudes, as there is no prominent scattering.
Red
is least scattered by fog or smoke. So, it can be seen in the same colour at a
distance. Therefore, red colour is used in Danger signal lights.
Colour of the Sun at Sunrise & Sunset
The blue colour of the
sky and the reddish appearance of the Sun at the sunrise
& sunset can be understood by a demonstration using a set up given below:
o Here,
white light from a strong source (S) passes in the following sequence:
White light → converging lens (L1) → formation
of parallel beam of light → transparent glass tank (T) containing 2 L of clear
water → circular hole (c) in a cardboard → converging lens (L2) → sharp
image of the circular hole forms on a screen (MN).
o Now
dissolve 200 g sodium thiosulphate (hypo) in water in the tank. Add 1 -
2 mL conc. H2SO4 to the water.
o Fine
microscopic sulphur particles precipitate in 2 - 3 minutes. As a result, blue light can be
observed from the three sides of the glass tank. This is due to scattering
of short wavelengths by colloidal sulphur particles.
o Observe
the colour of the transmitted light from the fourth side of the glass tank
facing the circular hole. It is observed at first the orange red colour
and then bright crimson red colour on the screen.
Light from the Sun near the horizon passes through thicker layers of air and larger distance in the earth’s atmosphere. So, the blue light & shorter wavelengths are scattered away by the particles. Therefore, only the light of longer wavelengths reaches our eyes. So the Sun appears reddish.
Light from the Sun overhead travels relatively shorter distance. At noon, the Sun appears white as only a little of the blue and violet colours are scattered.
